MySQL: Set connMaxLifetime value based on wait_timeout

**Why is this needed**: Better user experience **What would you like to be added**: This is from @ImraanGoonner > What I understand in this issue is that this is caused by an issue in the underlying library: https://github.com/go-sql-driver/mysql/issues/1393 and this is mitigated by setting connMaxLifetime to a value lower than wait_timeout. > From out engineering documents, it's recommended by DBAs for clients to set their own wait_timeout in their session based on their needs, and I'm not sure how this is handled in Grafana client but I think the default Grafana behavior can be improved as this is an issue that would impact all MySQL database connection and remove the need for users to tune their data source configurations. > What comes into my mind is when connMaxLifetime is not specified in the data source configuration, Grafana can either fetch wait_timeout from the session configuration, and set connMaxLifetime based on that value, or try to adjust wait_timeout of its MySQL session (as the variable is configurable per session) **Who is this feature for?** End users.